APPLIED THERMAL ENGINEEING 15ME52T UNIT-II STEAM BOILERS Steam generators are used to produce high pressure superheated steam. A steam generator is complex combination of economizer, boiler, super heater, reheater and air preheater. Boiler is a portion of the steam generator but it is used to mean the whole steam generator. DEFINITION OF BOILER A steam generator or a boiler is a closed steel vessel which generates the steam by transferring heat, produced by burning of fuel, to water. FUNCTION OF BOILER Following are the functions of steam boiler: 1. To transfer the heat produced by combustion of the fuel to water to generate the steam. 2. To supply the steam at the required constant pressure either dry or superheated. APPLICATION OR USES OF STEAM BOILERS (APPLICATION OR USES OF STEAM) Applications Steam boilers are as follows: (1) POWER GENERATION: Mechanical power can be generated by expanding steam either in a steam engine or in a steam turbine. If the engine/turbine is coupled to a generator, electric power can be generated. (2) HEATING: Steam is used in winter air conditioning of residential and industrial buildings. It is also used to heat water for hot water supply. (3) INDUSTRIAL PROCESSES: Steam is used for certain industrial processes such as sizing, bleaching etc., in textile mills, sugar factories, breweries etc., CLASSIFICATION OF BOILERS (1) BASED ON FLOW OF WATER AND HOT GASES (A) Fire tube boilers (B) Water tube boilers (2) BASED ON USE (A) Stationary boiler (B) Mobile boiler (3). BASED ON POSITION OF FURNACE (A) Internally fired boilers (B) Externally fired boilers GOVERNMENT POLYTECHNIC KAMPLI U-II : STEAM BOILERS Page 1 APPLIED THERMAL ENGINEEING 15ME52T (4). BASED ON AXIS OF BOILER SHELL (A) Horizontal boilers (B) Vertical boilers (5). BASED ON NUMBER OF TUBES (A) Single tube boiler (B) Multi tubular boiler (6). BASED ON METHOD OF CIRCULATION OF STEAM AND WATER (A) Natural circulation boiler (B) Forced circulation boiler (7). ACCORDING TO THE SOURCE OF HEAT Boilers are classified based on the nature of source of heat as follows: (A) Boilers which uses solid, liquid or gaseous fuels (B) Boilers which uses nuclear energy (C) Boilers which used electrical energy FIRE TUBE BOILERS: In fire tube boilers, the hot flue gases produced by combustion of fuels are passed through a tube or group of tubes around which water circulates. These boilers operate at moderate pressure (16 bars) and are suitable to generate 3-8 tons of steam per hour which is used in process heating. Eg., Cochran boiler, Lancashire boiler, Cornish boiler, Locomotive boiler etc., WATER TUBE BOILERS: In water tube boilers, water circulates inside the tubes while the hot flue gases produced by combustion of fuel pass over them externally. These boilers operate at very high pressure and are used for power generation. Eg., Babcock and Wilcox boiler, Stirling boiler, Yarrow boiler etc., STATIONARY BOILERS: If the boilers are used at one place only they are termed as stationary boilers. These boilers are used for either process heating in industries or for power generation in steam power plants. Eg., Babcock and Wilcox boiler, Fluidized bed combustion(FBC) boiler etc., MOBILE BOILER: If the boilers are portable they are known as mobile boilers. Such boilers are use in locomotive and ships. Eg., Locomotive boiler, Scotch marine boiler etc., GOVERNMENT POLYTECHNIC KAMPLI U-II : STEAM BOILERS Page 2 APPLIED THERMAL ENGINEEING 15ME52T INTERNALLY FIRED BOILERS: If the furnace is located inside the boiler shell, then the boilers is termed as internally fired boiler. Most of the fire tube boilers are internally fired. EXTERNALLY FIRED BOILERS: If the furnace is located outside the boiler shell, then the boilers is termed as externally fired boilers. Water tube boilers are always externally fired. HORIZONTAL BOILERS: If a boiler shell is placed with its axis horizontal, then it is called horizontal boiler. Eg., Cornish boiler, Locomotive boiler, Scotch marine boiler, Babcock and Wilcox boiler etc., VERTICAL BOILERS: If a boiler shell is placed with its axis vertical, then it is called vertical boiler. Eg., Cochran boiler SINGLE TUBE BOILER: In a single tube boiler there is only one fire or water tube. Eg., Cornish boiler MULTI TUBULAR BOILER: In a multi tubular boiler there are two or more fire tubes or water tubes. Eg., Lancashire boiler, Locomotive boiler, Cochran boiler, Babcock and Wilcox boiler etc., NATURAL CIRCULATION BOILER: In a natural circulation boiler, circulation of water is by natural convection currents which are set up during heating of water. All conventional boilers are natural circulation boilers. FORCED CIRCULATION BOILER: In a forced circulation boiler, the circulation of water is effected by mechanical pump which forces the water through the tubes. Since water is circulated rapidly under pressure, the heat transfer is maximum and steam generation is faster. Modern high pressure boilers employ the forced circulation system. Eg., Lamont boiler, Benson boiler, Velox boiler etc., COCHRAN BOILER It is a multi tubular, internally fired, vertical, natural convection, portable, fire tube boiler. It has boiler shell of about 3 meter diameter and 6 meter high. It has a steam generation capacity of 3500 – 4000 Kg/Hr and can generate steam at a pressure of about 10 to 20 bars. CONSTRUCTION: The boiler consists of a vertical, cylindrical shell and a fire box. Both the shell and fire box are hemi- spherical. The hemi-spherical crown of the shell gives maximum strength to withstand steam GOVERNMENT POLYTECHNIC KAMPLI U-II : STEAM BOILERS Page 3 APPLIED THERMAL ENGINEEING 15ME52T pressure inside the shell. The hemi-spherical crown of the furnace is useful to with stand the intense heat in the boiler furnace and to deflect the hot flue gases. The fire box and combustion chamber is connected by means of an elliptical flue tube. The flue gases from the combustion chamber flow to the smoke box through a number of fire tubes. These tubes are generally have 60 – 80 mm diameter and are 160 – 170 in numbers. The gases from the smoke box pass to the atmosphere through a chimney. The combustion chamber is lined with fire bricks on the shell side. A manhole near the top of the crown in the shell is provided for cleaning. The boiler is also fitted with mounting like steam stop valve, blow down valve, feed check valve, water level indicators, fusible plug, pressure gauge etc., WORKING: The boiler shell is filled with water using feed pump up to the required level. Water fills the shell completely covering the furnace, combustion chamber and the flue tubes. Coal is burnt in the furnace. The products of combustion rise in the fire box and get deflected towards the combustion chamber through elliptical flue tube. The hot flue gases from combustion chamber enter the smoke box via flue tubes. The gases from the smoke box are discharged to atmosphere through chimney. The hot gases while passing through the flue tubes transfer their heat to the water which is converted into steam. The steam thus formed is accumulated in the steam space. GOVERNMENT POLYTECHNIC KAMPLI U-II : STEAM BOILERS Page 4 APPLIED THERMAL ENGINEEING 15ME52T MERITS: 1. It occupies less floor space 2. It is portable and easy to transport and install. 3. It is simple in design and construction. 4. Due to flexibility in burning, any type of fuel can be used. DEMERTIS: 1. Rate of steam generation is low. 2. Cleaning and inspection is difficult due to vertical design. 3. It is suited only for small capacity requirement. BABCOCK AND WILCOX BOILER It is a horizontal, externally fired, natural circulation, stationary, water tube boiler. It has a steam generation capacity of 2 – 20 tons/Hr and can generate steam at a pressure of about 40 – 44 bars.Since it is capable of generating large quantities of steam at high pressure, therefore they are extensively used in thermal power plants. CONSTRUCTION: It mainly consists of four parts 1) Water and steam drum 2) Water tubes 3) chain grate stoker 4) Super heater tubes. Water and steam drum is suspended from iron girders (supporting beam) resting on the iron column (not shown in figure). A number of inclined water tubes, of about in 100 mm diameter at relatively low inclination, are connected at right angle to end boxes called ‘headers’. A mud box is provided just below the down take header. Any sediment in the water will settle down in the mud box and is blown off from time to time through blow off valve. The chain grate stoker is provided at the front end below the uptake header. The coal, fed on to one end of the grate, is burned on the moving grate in the furnace and the residual ash falls at the other end of the grate into ash pit. The boiler is provided with a super heater. The super heater consists of a number of U tubes secured at each end to the horizontal connecting boxes and placed in the combustion chamber below the boiler drum. The upper box of the super heater tubes is connected to a T (Tee) tube and upper branches of the T being situated in the steam space in the drum. The lower box of the super heater tubes is connected to steam stop valve mounted over the drum through a vertical tube passing outside the drum. GOVERNMENT POLYTECHNIC KAMPLI U-II : STEAM BOILERS Page 5 APPLIED THERMAL ENGINEEING 15ME52T WORKING: Water is introduced into the drum through feed valve. Constant water level is maintained in the boiler drum. The water descends at the rear end into the down take header and passes up in the inclined water tubes, uptake header and in the tubes connecting the uptake header and drum.